Babka recipe--Inside the Jewish Bakery

This book is great fun but the recipe for chocolate babka loaf is giving me fits. I've made it three times and each time the loaves emerge from the oven fully risen, glories to behold. And then as they cool they sink back into themselves until they become very tasty flat cakes. Is it me? The recipe? Something else entirely? I've followed the instructions to the letter. All suggestions gratefully received.